1. Field of Invention
The invention relates to a recovery process applied to a particulate filter that purifies exhaust gas discharged from an internal combustion engine, for example, a diesel engine and the like.
2. Description of Related Art
The process for reducing particulate matters (hereinafter referred to as PM), for example, soot and the like discharged from a vehicle driven by an internal combustion engine into atmosphere has been increasingly desired. There has been introduced a well known particulate filter (hereinafter simply referred to as a filter) that traps the PM discharged from the internal combustion engine disposed in an exhaust passage of the internal combustion engine in response to the aforementioned needs.
As more and more PM is trapped by the filter, the filter tends to have clogging, and accordingly the exhaust resistance increases, thus reducing outputs of the internal combustion engine. It is necessary to perform a filter recovery process to remove the PM trapped and accumulated on the filter through oxidization. Generally the filter recovery process is performed by injecting a fuel in the expansion stroke or the exhaust stroke during the operation of the internal combustion engine, that is, performing a post injection. Then the filter is brought into an oxidizing atmosphere (in excess of oxygen) while being heated until its temperature becomes 650° C. at which the PM can be oxidized. It is well known that the amount of the PM accumulated on the filter is estimated based on the differential pressure of the filter, and the filter recovery process is performed when the estimated accumulated PM amount exceeds a predetermined value.
The filter may trap the engine oil, certain substance contained in the fuel, or ash such as the oxidized scale in addition to the PM as the internal combustion engine is operated. As the trapped ash amount is increased, the differential pressure of the filter becomes higher.
If the accumulated PM amount is estimated based on the differential pressure of the filter, the accumulated ash amount may be measured as the accumulated PM amount. Accordingly it is determined that the amount larger than the actual amount of the accumulated PM is estimated. The filter recovery process is then performed even if the actual accumulated PM amount is small. This may reduce the time interval for performing the filter recovery process, thus deteriorating the fuel efficiency.
In JP-A-2003-83036, the ash amount that resides in the filter is derived from the differential pressure of the filter immediately after the filter recovery process. The derived residual ash amount is subtracted from the accumulated PM amount that has been estimated based on the differential pressure. The subsequent filter recovery process is started when the resultant accumulated PM amount exceeds a predetermined value.
As the amount of the ash trapped on the filter increases, the capacity of the filter for trapping the PM is decreased. In the aforementioned technology that starts the filter recovery process when the trapped PM amount obtained by excluding the ash amount from the PM amount estimated based on the differential pressure exceeds the predetermined value, the accumulated PM amount per unit volume is increased to be larger than that in the case where no ash is accumulated. This may cause the temperature of the filter to increase excessively owing to the oxidizing reaction of the PM upon recovery of the filter. Accordingly, the excessive temperature increase may melt or damage the filter.